1. Field of the Invention
The invention relates to an arrangement for processing auxiliary signals in the form of spatially separated signalling signals and pilot signals for a given number of main information signals for transmitting these auxiliary signals together with the main information signals in an FDM format.
The invention also relates to an arrangement for processing auxiliary signals of a given number of main information signals, for spatially separating and recovering these auxiliary signals in the form of signalling signals and pilot signals, which are applied to the arrangement together with the main information signals in an FDM format.
2. Description of the Prior Art
As known, an FDM signal for speech signals is composed of a plurality of FDM channels which each cover a bandwidth of 4 KHz. Each of these channels accommodates a speech channel for the transmission of a speech signal. Herein this speech signal has a bandwidth of 3.1 kHz and the distance between two successive speech channels in the FDM signal is 900 Hz. The frequency spaces of 900 Hz which are each time present between two successive speech channels are utilized for transmitting signalling signals and, possibly, pilot signals. More in particular an auxiliary channel is added to each speech channel for transmitting the signalling signals associated with this speech channel. It should be noted that the bandwidth of such signalling signals is only a few tens of hertz (for example 20 Hz).
The location described above of the signalling signals with respect to the associated speech signal is also indicated by out of band signalling. This in contrast with the so-called in-band signalling in which the signalling signals are located within the frequency band of the speech signal.
The signalling signals used in practice are all in the form of a series of pulses having a repetition frequency of, for example, 10 Hz. To accommodate these signals in the auxiliary channels of the FDM signal each of these signalling signals is filtered by means of a low-pass filter and thereafter modulated on a carrier of a suitably chosen frequency. In the reverse case, to separate the signalling signals spatially again from the FDM signal and to obtain the signalling signals in base band, the FDM signal is applied to a plurality of parallel channels. In each of these channels the FDM signal is demodulated with a carrier of a suitably chosen frequency. From each of the demodulated signals obtained in this way a signalling signal is again selected by means of a low-pass filter. For the proper operation of both arrangements the cut-off frequencies of the low-pass filters used must be low, for example 50 Hz. However, this results in very bulky and expensive filters.
With an FDM signal one or more pilot signals are also transmitted together with the signalling signals. For a primary FDM group which is located in the frequency band of 60 - 108 kHz one or more pilot signals are accommodated with the frequencies 84.080 kHz, 84.140 kHz or 104.80 kHz respectively. At the receiver side of the FDM transmission system the level of such a pilot signal is used for automatic volume control of the speech signals. In view of the accuracy which is required for this volume control both the frequency and the amplitudes of these pilot signals must be very stable whilst at the receiving side of the FDM transmission system extremely selective filters must be used to select these pilot signals from the FDM signal. Such a filter has, for example, a bandwidth of 20 Hz, whereas its intermediate frequency coincides with the pilot frequency. In practice quartz oscillators are used for producing the pilot signals whilst very expensive quartz filters are used for selecting the pilot signals from the FDM signal.
In practice the above means that approximately 40% of the costs of an FDM multiplexer or demultiplexer is determined by the necessary transmission of signalling signals and pilot signals.